The present invention relates generally to antennas, and more particularly to patch antennas with capacitive elements.
Patch antennas are widely deployed in many devices, such as global positioning system receivers and cellular telephones, because they are small and lightweight. The basic elements of a conventional patch antenna are a flat radiating patch and a flat ground plane separated by a dielectric medium. One type of patch antenna, referred to as a microstrip antenna, may be manufactured by lithographic processes, such as those used for the fabrication of printed circuit boards. These manufacturing processes permit economical, high-volume production. More complex geometries, such as used for phased-array antennas, may also be readily manufactured.
In a common design for microstrip antennas, the ground plane and the radiating patch are fabricated from metal films deposited on or plated on a dielectric substrate. In many applications, it is desirable to have a patch antenna with a wide directional pattern and a wide operating frequency bandwidth. In the design of a microstrip antenna, there are dependencies between mechanical and electromagnetic parameters. The directional pattern increases as the size of the patch decreases. The length of a microstrip patch is equal to one-half the wavelength of the electromagnetic wave propagating in the dielectric substrate. The length of a microstrip patch may be reduced by using dielectrics with high permittivity. For antennas operating in the radiofrequency and microwave bands, however, dielectrics with high permittivities also have high densities, resulting in increased weight of the antenna. Similarly, the operating frequency bandwidth may be increased by increasing the thickness of the dielectric substrate, which again results in additional weight.
There have been various proposed designs for reducing the size and weight of patch antennas. For example, M. K. Fries and R. Vahldieck (Small microstrip patch antenna using slow-wave structure, 2000 IEEE International Antennas and Propagation Symposium Digest, vol. 2, pp. 770-773, July 2000) reported a microstrip patch antenna in which miniaturization is achieved by using a slow-wave circuit and a structure in the form of cross-shaped slots in the radiating patch and ground plane. Such an antenna has a simple design and light weight, but the presence of slots prevents the installation of a printed circuit board with a low-noise amplifier on the antenna, a common design architecture. What are needed are patch antennas with small size, light weight, wide directional pattern, and wide operating frequency bandwidth. Patch antennas which permit the ready integration of auxiliary electronic assemblies, such as low-noise amplifiers, are further advantageous.